1. Field of Invention
The present invention is related to a zinc-plated metallic material, more particularly to a corrosion-resistant coating applied on a metallic material which has improved corrosion-resistance after heating. Mainly, the coating according to the present invention is applied on a steel sheet and comprises mainly Zn.
2. Description of Related Arts
The ordinary rust-proofing method of steel materials is Zn plating. It is required particularly in the Zn-plated steels used in automobiles to enhance the heat-resistance of the Zn plating.
Japanese Unexamined Patent Publication No. 60-141898 discloses a composite zinc plating, in which oxides of Al, Ti, Si and the like are dispersed so as to enhance the a corrosion resistance. The composite components used in this Zn coating are, however, chemically stable oxides and hence exhibit merely a physical protecting function. The corrosion resistance of this coating is therefore unsatisfactory.
Therefore the use of a compound having a chemically inhibiting function as the composite component has been tried, to attain satisfactory corrosion-prevention with a small amount of addition.
Such composite plating is disclosed, for example, in Japanese Unexamined Patent Publication No. 60-125395 and consists of zinc and a soluble aluminum oxide, whose Al.sup.3+ ions realizes the inhibiting function and attains an improved corrosion-resistance under such an environment where the aluminum oxide is dissolved.
Many automobile parts are exposed under a high-temperature condition due to heat emitted from the engine and the gas-exhausting system. The post-heating corrosion-resistance is, therefore, required for such parts. However, in the provision of the conventional composite zinc plating, no consideration has been made regarding this point in the conventional composite zinc plating, that is, to the deterioration in corrosion-resistance which follows heating. Even in the case of acid-soluble aluminum oxide dispersed in the Zn plating layer, crystallization of the Al compound proceeds due to heating and is converted to an insoluble form, thereby deteriorating the corrosion-resistance.
When the Zn-plated steel sheet is used in such a manner that the Zn plating layer is exposed, zinc oxide, which is rust of zinc, readily forms. This rust is referred to as white rust. The Zn-plated steel sheet is, therefore, conventionally subjected to chromating so as to prevent the formation of white rust. The chromating method consists of dipping a steel sheet in an aqueous solution containing hexavalent chromium. Various chromate coatings, such as lustrous, yellow and green coatings are formed which, however, have low heat-resistance. When these coatings are heated to a temperature of approximately 80.degree. C. or more, such defects, as cracking, peeling and the like, are formed on the chromate coating, with the result that the corrosion resistance is drastically lowered.
For the reasons as described above, the corrosion-resistance of the chromate coating is insufficient for using a chromated material for the parts used in the engine room of an automobile. Another problem of chromate coating lies in its appearance due to color, such as yellow or interference color of hexavalent chromium. A countermeasure against this problem is to reduce the deposition amount of chromium to such a level that a virtually colorless coating is formed. This countermeasure, however, causes another problem in that the corrosion-resistance of the chromate coating is reduced.